GEOL 104 Dinosaurs: A Natural History

Fall Semester 2006The Dinosaur Family Tree: What is a dinosaur?

Dinosauria defined as the most recent common ancestor of Iguanodon and Megalosaurus
and all of its descendants.

Dinosauria is divided into two clades, Ornithischia and Saurischia

Ornithischia ("bird hips"):

Defined as Iguanodon and all taxa closer to Iguanodon than to Megalosaurus

Diagnosed by presence of the predentary bone (an extra bone joining the two
anterior ends of the dentary, forming a beak) and five or more sacrals

All known ornithischians were most likely herbivores, with leaf-shaped teeth

Most primitive known ornithischian is Pisanosaurus from the Ischigualasto
Formation (early Late Triassic) of Argentina: about 1 m long.

All other, more derived ornithischians have a pubis that points backwards and ossified
tendons along the top of the vertebrae.

Other primitive ornithischians include Early Jurassic Lesothosaurus and the
Heterodontosauridae:

Heterodontosauridae ("different toothed"):

Retain the long grasping hands (in most other ornithischians the hands are short and are non-grasping)

Short, deep, powerful skulls

Long considered a type of ornithopod, and also suggested as sister group to Marginocephalia

Oldest is a possible Late Triassic Argentine specimen; greatest diversity is in Early Jurassic
of southern Africa; last known species in early part of Early Cretaceous

Small (1 m long) bipedal animals

Advanced ornithischians include two major clades: Thyreophora, and Neornithischia.

Thyreophora ("sheild bearer):

Armored dinosaurs: diagnosed by presence of rows of dermal ossifications

Early thyreophorans small (1 m long) bipedal animals; later forms much larger and
quadrupedal, with heavier armor

Neornithischians are diagnosed by asymmetrical enamel on their teeth, giving them an
ever-sharpening surface. Among the neornithischians are various primitive forms like
Early Jurassic Strombergia and Middle Jurassic Agilisaurus and Hexinlusaurus,
plus the advanced diverse Ornithopoda and Marginocephalia.

Ornithopoda ("bird foot"):

Specialized jaws (similar features seen in advanced heterodontosaurids):

Premaxilla lower margin ventral to maxilla lower margin

Jaw articulation ventral to maxilla tooth row

Early ornithopods small (1 m long) bipedal animals; later forms increase in size and
retain bipedality, but develop more sophisticated chewing jaws

Marginocephalia ("ridge head"):

Characterized by shelf of bone extending back over occipital region of skull

Primitive forms are bipedal; in one branch derived forms become very large and obligate quadrupeds

The other major clade of dinosaurs is Saurischia:

Defined as Megalosaurus and all taxa closer to Megalosaurus than to
Iguanodon

Diagnosed by:

Special articulations between dorsal vertebrae (technically called hyposphene-hypantrum
articulations)

Complex chambers in vertebrae

The most primitive saurischians are the Herrerasauria:

Carnivorous bipedal dinosaurs, up to about 4 m long

Most completely known form is Herrerasaurus of the Ischigualasto Fm.
(early Late Triassic of Argentina)

Has longer clutching fingers, shared with theropods (see below)

Has an intramandibular joint between the dentary and postdentary bones

Absorbs the stresses of struggling prey

Also found in theropods

Herrerasaurs were once thought to be true theropods, but have only two sacrals (a
pre-dinosaur feature), have manual digit III longer than manual digit II (a pre-eusaurischian feature),
and have relatively short necks (another pre-eusaurischian feature). Recent studies place them
as a group primitive saurischians rather than true theropods.

Also in Ischigualasto Fm., another very primitive saurischian: Eoraptor. Overall,
one of the least-transformed of the Triassic dinosaurs, and probably the best approximation known
to the ancestral dinosaur form. Recent studies place it outside of Eusaurischia, but within Saurischia.

The remaining part of Saurischia, called Eusaurischia, is recognized by:

Elongate necks where posterior cervicals are longer than anterior cervicals

Manus with an enlarged digit I ungual (thumb claw)

Manual digit II is the longest in the hand

Some eusaurischians retain the primitive carnivorous condition; others have
leaf-shaped teeth and were herbivores.

Eusaurischia is divided into two major clades, Sauropodomorpha and Theropoda.

Sauropodomorpha ("sauropod form"):

Characterized by:

Enlarged nares

A VERY elongated neck

Leaf-shaped teeth

Herbivores (early forms maybe omnivores)

Primitive forms are facultative bipeds; later forms were so large they were obligate
quadrupeds

Theropoda ("beast foot"):

Characterized by extremely hollow limb bones

Most retain the primitive condition of bladelike, serrated teeth,
indicating they were carnivores

All, from smallest to largest, were obligate bipeds

Has an intramandibular joint between the dentary and postdentary bones

Promaxillary fenestra (an extra opening in the maxilla between the naris and
the antorbital fenestra)

Five or more sacrals

Furcula (clavicles fused into wishbone)

Manual digit V lost: hand has only four fingers

Pedal digits I and V reduced, so foot is functionally three-toed

By Late Triassic good fossils are known of Sauropodomorpha, Eoraptor,
Herrerasauridae, and Theropoda. Ornithischia only poorly known then.

With the Triassic-Jurassic extinction event, most potential dinosaur rivals (therapsids, pseudosuchians, etc.)
were removed, and Dinosauria became the dominant group on land: the Fourth Amniote Radiation.